The island's geography comprises relatively low-lying mountains surrounding a central plain, with several navigable rivers extending inland. Its lush vegetation is a product of its mild but changeable climate which is free of extremes in temperature. It was covered by thick woodlands until the Middle Ages. As of 2013, the amount of land that is wooded in Ireland is about 11% of the total, compared with a European average of 35%. There are twenty-six extant mammal species native to Ireland. The Irish climate is influenced by the Atlantic Ocean and thus very moderate, and winters are milder than expected for such a northerly area, although summers are cooler than those in Continental Europe. Rainfall and cloud cover are abundant.

The names Ireland and Éire derive from Old IrishEriu. This in turn comes from the Proto-Celtic*Iveriu (compare WelshIwerddon), which is also the source of Latin Hibernia. Iveriu derives from a root meaning 'fat, prosperous'.

During the last glacial period, and up until about 10,000 BC, most of Ireland was periodically covered in ice. Sea levels were lower and Ireland, like Great Britain, formed part of continental Europe. By 16,000 BC, rising sea levels due to ice melting caused Ireland to become separated from Great Britain. Later, around 6000 BC, Great Britain itself became separated from continental Europe. The earliest evidence of human presence in Ireland is dated at 10,500 BC, demonstrated by a butchered bear bone found in a cave in County Clare. It is not until about 8000 BC, however, that more sustained occupation of the island has been shown, with evidence for Mesolithic communities around the island. These Mesolithic communities lived as hunter-gatherers across the island until about 4000 BC.